1. Field of the Invention
The present invention relates to a MRI (magnetic resonance imaging) apparatus and a magnetic resonance imaging method which excite nuclear spin of an object magnetically with a RF (radio frequency) signal having the Larmor frequency and reconstruct an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which make it possible to measure a flow velocity of fluid.
2. Description of the Related Art
Magnetic Resonance Imaging is an imaging method which excites nuclear spin of an object set in a static magnetic field with a RF signal having the Larmor frequency magnetically and reconstruct an image based on NMR signals generated due to the excitation.
The PS (phase shift) method has been used commonly as a measuring method of a blood flow velocity in the field of magnetic resonance imaging. The PS method also called the phase contrast MRA (magnetic resonance angiography) is one of the MRA imaging methods which images blood flow based on the phase information of spins (see, for example, Japanese Patent Application (Laid-Open disclosure) No. 2002-165771). Specifically, when the gradient magnetic field is applied as a bipolar gradient, the phase of the static spins does not change over the application of the gradient magnetic field but the phase of the moving spins in blood flow shifts over the application of the gradient magnetic field. The phase shift of the spins depends on the intensity of the applied gradient magnetic field, the application period and the spin velocity. The blood flow velocity and the blood flow direction can be calculated based on the phase information of the spins using this relation.
In addition, the t-SLIP (Time-SLIP: Time Spatial Labeling Inversion Pulse) method has been known in the field of magnetic resonance imaging as one of the non-contrast-enhanced MRA methods that can selectively depict a blood vessel image without a contrast medium. In the t-SLIP method, the t-SLIP pulse for labeling is applied in accordance with the t-SLIP sequence and blood flowing into an imaging area is labeled. That is, the t-SLIP sequence is an imaging sequence with applying an ASL (arterial spin labeling) pulse for tagging blood flowing into an imaging section so that the tagged blood is selectively imaged or suppressed. According to this t-SLIP sequence, the signal intensities of only blood reaching the imaging section after the TI (inversion time) can be selectively emphasized or suppressed. By setting the spatial position where the t-SLIP pulse is applied and the TI corresponding to the waiting time from an application timing of the t-SLIP pulse to an imaging properly, various blood vessels can be selectively imaged and suppressed. Moreover, a technique that performs a pre-scan called the TI-prep, for acquiring data with a gradual TI change, prior to an imaging scan under the t-SLIP method has been designed as a method to acquire the optimum TI in the case where the spatial position where the t-SLIP pulse is applied is fixed (see, for example, Japanese Patent Application (Laid-Open disclosure) No. 2003-70766).
However, when a flow velocity is measured by the conventional PS method, a complicated work that the ROI (region of interest) is set to specify the position of a fluid to be measured is needed.
Especially, the PS method is a technique to image the phase difference of signals. Therefore, PS method can not acquire a blood vessel image with an improved contrast compared with a technique to image the intensity difference of signals. Consequently, there is a problem that searching for the blood vessel to be a measurement target of the flow velocity in a blood vessel image acquired by the PS method is difficult. In addition, there is a problem that the imaging time becomes long in case where a flow velocity is measured by the conventional PS method.
Meanwhile, when imaging is performed by the t-SLIP method, a blood flow velocity is required as an imaging condition. For this reason, prior to an imaging by the t-SLIP method, the development of method to acquire a flow velocity of a fluid such as a blood flow more easily in a shorter time is required.